1. Field of the Invention
The present invention relates to an exposure apparatus in which a liquid immersion area is formed in an optical path space disposed on a side of a substrate, an optical element which is usable for the exposure apparatus, an optical system which includes the optical element, an exposure method which is based on the use of the exposure apparatus, and a method for producing a microdevice based on the use of the exposure apparatus.
2. Description of the Related Art
Those having been hitherto suggested as the exposure apparatus of this type include, for example, an exposure apparatus described in International Publication No. 99/49504. The exposure apparatus of this patent document is provided with an illumination optical system which irradiates a mask such as a photomask or a reticle with an exposure light beam radiated from an exposure light source, and a projection optical system which projects an exposure pattern formed on the mask onto a substrate such as a glass plate or a wafer coated with a photosensitive material (resist). The illumination optical system and the projection optical system have barrels respectively. At least one optical element (for example, a lens) is accommodated in each of the barrels.
In the case of the exposure apparatus described in International Publication No. 99/49504, a liquid (pure water), which has a refractive index higher than that of the gas, is supplied to the optical path space as the space formed between the projection optical system and the substrate to form a liquid immersion area in order to respond to the advance of the high density of the device and the advance of the fine pattern to be formed on the substrate. Therefore, the exposure light beam, which is allowed to pass through the projection optical system, is radiated onto the substrate after passing through the liquid immersion area.
In the case of the exposure apparatus described in International Publication No. 99/49504, the surface of the optical element arranged on the side nearest to the substrate in the projection optical system, which is opposed to the substrate, makes contact with the liquid. Therefore, for example, if a large amount of the liquid is unexpectedly supplied, for example, by a liquid supply control mechanism, the following possibility arises. That is, a part of the liquid disposed in the liquid immersion area may travel along the side surface of the optical element to make inflow into the barrel after passing through the gap between the optical element and a holder (holding member) for holding the optical element on the barrel. It is demanded that the optical element, which is used under the severe condition of the liquid immersion exposure, maintains appropriate characteristics for a long period of time.